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Durham University

Department of Earth Sciences

Profile

Publication details for Prof Ken McCaffrey

Wilson, P.I.R, McCaffrey, K.J.W. & Holdsworth, R.E.H. (2019). Magma-driven accommodation structures formed during sill emplacement at shallow crustal depths: The Maiden Creek sill, Henry Mountains, Utah. Geosphere 15(4): 1368-1392.

Author(s) from Durham

Abstract

In areas of exceptional exposure, upper-crustal intrusions and their immediate
wall rocks commonly preserve direct evidence of the emplacement,
magma flow pathways, and strains associated with the intrusion process.
Such excellent exposure is displayed by the Paleogene Maiden Creek intrusion—
a small satellite body related to the Mount Hillers intrusive complex,
Henry Mountains, Utah. An intermediate plagioclase-hornblende porphyritic
magma was intruded into the Entrada Sandstone Formation at an estimated
depth of ~3 km. The southern part of the intrusion is overlain by the newly
identified Maiden Creek shear zone (MCSZ): a subhorizontal, top-to-the-WNW
detachment formed at the contact with the overlying sandstone country rocks.
From observations of both syn-emplacement deformation and the exposed
intrusion geometries, it is proposed that the southern Maiden Creek intrusion
comprises westerly-derived, inclined sill sheets. Host-rock sandstones were
sandwiched (~E–W constriction) between these intrusive bodies beneath the
MCSZ. It is proposed that the MCSZ is a syn-emplacement magma-driven
accommodation structure, with a shear sense antithetic to the magma flow
direction, which played a critical role in accommodating the westerly-derived
sill intrusion. Our results show that inelastic syn-emplacement deformation
structures, such as the MCSZ, are very important in the accommodation of
magma in the subsurface. Such small structures are unlikely to be imaged by
seismic-reflection surveys, highlighting the importance of detailed field studies
in our understanding of intrusion geometry and emplacement mechanisms.